1. Field of the Invention
The invention relates to tissue processing cassettes, and, in particular, to tissue processing cassettes adapted for preparing very small tissue specimens for histological examination.
2. Description of the Prior ART
Tissue processing cassettes used for preparing biological tissue specimens for histological examination are well known. Tissue processing cassettes typically include a rectilinear, open-topped box-like base having a perforated bottom wall and a removable perforated cover, or lid. The cassettes are generally fabricated of a moldable plastic that resists damage or reaction from processing solvents and acids or reaction with the tissue specimen. A tissue specimen is typically placed within an enclosure formed by the base and lid and then processed in various solutions appropriate to the tissue and the examination.
The tissue processing steps are generally done in batches with other tissue specimens in a tissue cassette processing container, wherein each tissue specimen is enclosed within its own tissue processing cassette. The processing fluids pass through the perforated bottom walls and lids of the tissue processing cassettes in the processing container. Such treatments can include, for example, fixing, dehydrating, clearing, and infiltrating the tissue specimen with molten paraffin. After the aforesaid processes are completed, the lid is typically removed and the specimen removed to a mold. The mold is also adapted to receive the base, which is typically rectangular-shaped, in a recess in the top of the mold above the specimen. The mold is then filled through the perforations in the base with liquid paraffin or another tissue encasing material. After the paraffin sets, the base, having a block of solidified paraffin encasing the tissue specimen attached thereto, is mounted in a microtome. A pan of the paraffin block, including the specimen, projects beyond the base whereby a section of the tissue specimen can be sliced off for histological examination.
The perforated bottoms and lids of the tissue processing cassettes are typically rigid webs having openings at least about 1.0 mm in size in order to allow the processing fluids to flow therethrough. A serious problem arises where the tissue specimen is about the size or smaller than the size of the pores in the base or the lid. The specimen could be lost or washed into a neighboring cassette. Even if the tissue specimen is larger than the pore size, parts of a specimen smaller than the pore size can break away from a larger tissue specimen and migrate to another capsule, thereby contaminating that specimen.
Special procedures are typically employed to resist cross-contamination or loss of very small specimens when preparing such specimens for histological examination. Special procedures can include wrapping the specimen in filter paper or in a sponge, or placing it in a small bag of woven material having a fine mesh. The processes of wrapping and unwrapping or bagging and unbagging of the tissue specimen can distort the specimen, contaminate the specimen, or increase the chance of tissue loss.
U.S. Pat. No. 4,557,903, to McCormick, discloses a tissue processing cassette which includes a pair of interlocking frames, each of the frames having a porous web having a porosity between about 25% and about 75%, preferably fabricated from a non-woven nylon. The web extends across a large rectangular central opening in each of the bottom frame and top frame. The non-woven nylon webbing resists passage of small tissue specimens from one cassette to another neighboring cassette. The non-woven nylon screen must be cut to permit passage of the liquid paraffin during the step of encasing the specimen in paraffin. In addition, when preparing very small tissue specimens for histological examination, the specimen may be lost in a comer of the opening in a frame, making it difficult to locate.
Therefore, there is a need for an alternative to prior art tissue processing cassettes that will permit small tissue specimens, generally less than 1.0 mm in size, to be prepared for a histological examination without cross-contaminating nearby specimens or distorting or losing the specimen from the tissue processing cassette during the processing.